Congestive heart failure, or heart failure, is a major cause of morbidity and mortality. Approximately 5 million people in the United States suffer from heart failure, with approximately 500,000 new cases diagnosed annually. In many cases, congestive heart failure patients also suffer an arrhythmia further reducing the heart's efficiency.
One treatment option for such patients involves implantation of a cardiac resynchronization therapy (CRT) device, also known as a biventricular pacing device. In a normal heartbeat, the two atrial chambers of the heart contract in unison, pumping blood into the two ventricular chambers. Less than a second later, the two ventricular chambers contract in unison, pumping the blood out of the heart and throughout the body. Some heart failure patients have an electrical delay between the ventricles, such that the two ventricular chambers no longer contract at the same time. The result is a substantial reduction in cardiac output. For example, in a normal heartbeat more than half of the blood in the left ventricle is pumped out with each heart beat. In other words, the “left ventricular ejection fraction” or “LVEF” is greater than 50%. In contrast, some heart failure patients whose heartbeats are uncoordinated have an LVEF less than one-third. In these patients, two-thirds of the blood remains “unpumped” with each heartbeat.
The goal of CRT therapy is to restore coordinated pumping of the ventricles. This is accomplished by a device with separate electrical leads stimulating the two ventricles to contract simultaneously with every heartbeat. CRT devices have improved quality of life and have decreased mortality in some patients with moderate or severe heart failure; an LVEF less than or equal to 35%; and an echocardiogram indicating a slow depolarization of the ventricles (a “QRS” complex greater than 120 ms). Still, about 30% of heart failure patients who receive CRT fail to respond to treatment (see, for example, Jeevanantham et al. (2009) Cardiol. J. 16(3)197-209). As one author warned in 2007, “better criteria for identification of the optimal CRT candidate are urgently warranted” (Stellbrink (2007) Eur. Heart J. 28:1541-1542). Further, even among heart failure patients who receive CRT intervention, the rate of adverse events such as unplanned hospitalizations for heart failure and death is high (see, for example, Cleland et al. (2005) “The effect of cardiac resynchronization on morbidity and mortality in heart failure.” N. Engl. J. Med. 352(15):1539-49.